Double click the appengine-java-sdk-1.9.56.zip
file that you downloaded to extract the SDK to a directory of your
choice. It doesn't matter where you install the SDK but
you must add the directory to your %PATH%
environment variable. Find Environment Variables
in the Advanced System Settings dialog and then
add the directory to your %PATH% variable.

Set the javac compiler flags to generate the byte code level required by the App Engine Java
runtime you are using, per the tabs below:
Java 8 runtime

The App Engine Java 8 runtime can use either Java 1.7 or 1.8 bytecode level. You can use
either of these compiler flags:

-source 1.7 -target 1.7

or

-source 1.8 -target 1.8

Java 7 runtime

The App Engine Java 7 runtime requires Java 1.7 bytecode level. You can use either Java 7 or Java
8 JDKs to generate Java 7 byte code by using this compiler flag: